The present invention relates to a method and an apparatus for decanting liquids of all natures, notably waste water, through a so-called "pulsated" sludge bed.
In hitherto known decanting systems, the liquid to be treated, notably water, to which suitable reagents such as coagulating agents, flocculating agents, pH correctors, activated carbon, etc., have been added, is caused to circulate upwardly through the previously formed sludge bed which is adapted to act by contact with the water and to promote the settling or agglomeration of the precipitates thus formed which are retained within the sludge bed proper together with the fine particles of materials in suspension in the liquid to be treated. Conventionally, the liquid to be treated is introduced into the apparatus at a uniform rate of flow, or according to the method disclosed in U.S. Pat. No. 3,068,172 the liquid is introduced intermittently into the apparatus and spread throughout the lower portion of the settling tank. The sludge bed, thereby regularly pulsated from the bottom to the top thereof, is kept in an expanded condition throughout its mass, whereby an intimate contact is maintained between the liquid to be treated and the sludge. The liquid circulates upwardly at a certain average speed in the tank, and this speed should preferably be kept as high as possible, without however impairing the sludge cohesion for a predetermined amount of decanted water.
In pulsated sludge bed decanting processes, two parameters are important in the promotion of flocculation and coagulation, i.e. the concentration of matter in suspension in the sludge bed, which is subordinate to the decanting rate and to the pulsation parameters, and the average upward flow rate of the liquid to be treated through the sludge bed, which rate should advantageously be as high as possible without breaking the cohesion of the sludge.
The size of the settling tanks, and therefore the necessary investment costs, are closely dependent on the average upward flow rate of the liquid to be treated. This rate is obviously limited by the risk of carrying along sludge in the upward stream of liquid, and therefore causing a loss of sludge by entrainment.